Wavelength division multiplexing (WDM) systems are known in which plural optical signals or channels are transmitted over an optical fiber, with each channel being assigned a particular wavelength. Such systems typically include a plurality of receivers, each detecting a respective channel by filtering, for example, the remaining channels.
Optical channels in a WDM system or WDM channels are frequently transmitted over silica based optical fibers, which typically have relatively low loss at wavelengths within a range of 1525 nm to 1580 nm. WDM channels at wavelengths within this low loss “window” can be transmitted over distances of approximately 50 km without significant attenuation. For distances beyond 50 km, however, optical amplifiers are required to compensate for optical fiber loss.
Optical amplifiers have been developed which include an optical fiber doped with erbium. The erbium-doped fiber is “pumped” with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, erbium doped fiber amplifiers typically do not uniformly amplify light within the spectral region of 1525 to 1580 nm. For example, an optical channel at a wavelength of 1540 nm, for example, is typically amplified 4 dB more than an optical channel at a wavelength of 1555 nm. While such a large variation in gain can be tolerated for a system with only one optical amplifier, it cannot be tolerated in a system with plural optical amplifiers or numerous, narrowly-spaced optical channels. In these environments, much of the pump power supplies energy for amplifying light at the high gain wavelengths rather than amplifying the low gain wavelengths. As a result, low gain wavelengths suffer excessive noise accumulation after propagating through several amplifiers.
Accordingly, optical amplifiers providing substantially uniform spectral gain have been developed. In particular, optical amplifiers including a dynamic gain equalizer (DGE) are known, which can flatten an output spectrum of an optical amplifier. Various techniques are also known for controlling to DGE, such that the output power spectrum of an optical amplifier is substantially uniform. There is a need, however, for a method and apparatus that can control a DGE to rapidly converge to a spectrally uniform amplifier output.
In addition, conventional DGEs may be characterized as having a plurality “pixels”, each of which having an associated attenuation to adjust the optical power of a given channel. Typically, due to limitations of the DGE, the attenuation associated with one pixel may not exceed the attenuation of an adjacent pixel by a predetermined amount. Accordingly, there is also a need for a method and apparatus that controls those pixels that do not receive a channel, such that adjacent pixels that receive channels may impart a desired amount of attenuation.
Further, in many WDM systems, the spectral spacing between channels has been reduced in order to provide additional channels and realize systems with higher capacity. In such systems, the channel spacing may be 25 GHz, and a total of 160 channels may be transmitted. On the other hand, commercially available DGEs may have a different number of pixels, such as 128. Thus, there may not be a one-to-one correspondence between channels and pixels, such that operation of one pixel may interfere with the transmission of multiple channels. Accordingly, it is also desirable to appropriately control of the DGE pixels when the number of channels supplied to the DGE is different than the number of pixels of the DGE.